1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing and more specifically to a method and apparatus for improving the electrical characteristic of semiconductor devices.
2. Discussion of Related Art
Integrated circuits are made up of literally millions of active and passive devices such as transistors, capacitors and resistors. In order to provide more computational power and/or more storage capability in an integrated circuit, device features are reduced or scaled down in order to provide higher packing density of devices. An important feature to enable scaling of devices is the ability to form high quality, high dielectric constant films for capacitor and gate dielectrics.
High dielectric constant films are generally ceramic films (i.e., metal-oxides) such as tantalum pentaoxide and titanium oxide. When these films are deposited they tend to have vacancies at the anionic (oxygen) sites in the lattice. Presently these vacancies are filled by annealing the film in an oxygen containing gas. However, such an anneal causes the oxidation of the underlying polysilicon electrode in the case of a capacitor and the oxidation of the underlying silicon substrate in the case of a MOS transistor. Such an oxidation of the underlying silicon layer causes the formation of a silicon dioxide film in series with the high dielectric constant film which in turn reduces the effective dielectric constant of the combined film.
As such, in order to integrate a high dielectric constant film, such as tantalum pentaoxide, into a capacitor or MOS fabrication scheme, it has been found important to form a silicon nitride barrier layer between the high dielectric constant material and the silicon surface in order to prevent oxidation of the underlying silicon surface and a corresponding reduction in the effective capacitance of the device. The silicon nitride barrier layer, however, needs to be formed thin, between 10-25 .ANG., so that its low dielectric constant does not decrease the effective dielectric constant of the high dielectric constant film. Additionally, the silicon nitride film needs to be formed at low temperatures, less than 950.degree. C., in order to keep the thermal budget of the fabrication process low. Unfortunately, however, a thin silicon nitride layer formed at a low nitridation temperature creates a poor quality barrier to oxygen diffusion. Additionally, defects such as dangling bonds can be created at the silicon nitride/silicon surface interface. Such dangling bonds can lead to poor device performance such as increased leakage currents and reduced capacitance.
Thus, what is needed is a method of forming a thin robust silicon nitride barrier layer at a relatively low temperature and a method which can cure defects at the silicon nitride/silicon surface interface.